698 COLLEGE ZOOLOGY 



b. The Evolution of the Horse l 



One of the best methods of illustrating the value of studying 

 fossil animals is to give a brief description of a succession of con- 

 necting links such as are exhibited by the evolution of the horse. 

 The horses now inhabiting America are descendants of domesti- 

 cated animals which were brought to this country by the early 

 settlers from Europe, but in prehistoric times the ancestors of 

 our modern horse were native here; and some of the finest fossil 

 remains of these ancestors have been^ found in America. 



The evolution of the horse has been traced back through at 

 least twelve distinct stages extending through the Cenozoic Era 

 or the Era of Mammals. A brief description of five of these 

 stages together with Figure 552 will serve to illustrate the prin- 

 cipal changes that took place during this evolution. The 

 structural features that became modified during this era of about 

 3,000,000 years were such as to adapt the horse to life on the 

 open plains, where its food consisted of dry silicious grasses. 



The feet gradually lost the side toes, and only the middle toe 

 and splints of the second and fourth digits remain in our modern 

 horses. The limbs became longer, enabling the animal to move 

 about more rapidly; this change was correlated with an elonga- 

 tion of the head and neck, which was necessary in order to reach 

 the ground. The front teeth were modified as chisel-like crop- 

 ping structures, and the back teeth evolved from simple molars 

 into wonderfully effective grinding organs with tortuous ridges 

 of enamel and with supporting and protecting layers of dentine 

 and cement. During the later periods the molars elongated, 

 and thus became adapted for grinding the dry silicious grasses 

 which caused them to wear down more rapidly than the softer 

 vegetation. During this evolution the body gradually increased 

 in size from that of the earliest known form, which was about as 

 large as a domestic cat, to that of the horse of to-day. 



1 For a detailed account of this subject, see "The Evolution of the Horse" by 

 W. D. Matthew, Sup. to Am. Museum Journ., Vol. 3, 1903. Guide Leaflet, No. 9, 



